I wasn't very clear. I don't think it's the mass. From what I recall on a 50,000 sf pour I watched years ago, cracking is most often a result of not quite the right amount of water (too much, I think) in the mix and/or too fast a cure. Easier said than done. Watching the QAs get it right across multiple truckloads was interesting to watch. Every truck load got a slump test before it was allowed to unload. The QAs rejected a few every day, much to the anger of the truck drivers and provider. And then there was someone spraying down the pads daily for a week or two while it cured. We still got tiny cracks. And then the Corps had the contractor back out a month or so later to hammer out one pad with a test sample that failed a compression test.
I think you're right about the foundation but it needs to be bomb-proof. The compacted clay the last spillway was built on played a significant role in the its failure.